The present invention relates to a knocking control system for controlling knock control factors such as ignition timing in accordance with a generation state of knocking in an internal combustion engine.
Conventionally, there is proposed a system in which a peak value of a knock signal outputted from a knock sensor is detected so as to produce a knock decision level on the basis of the logarithmic conversion distribution of the peak values to decide the occurrence of the knocking by comparing the peak value with the knock decision level. For example, according to the Japanese Patent Provisional Publication No. 1-315649, the central value (median) V.sub.M of the distribution of the substantial logarithmic conversion values V of the peak values is detected to determine a value S corresponding to the standard deviation of the conversion value V so that the probability of V.sub.M /S.ltoreq.V.ltoreq.V.sub.M is 1/3 (33%). Further, the knock decision level V.sub.KD is produced in accordance with V.sub.KD =S.sup.3 .times.V.sub.M so as to decide the occurrence of the knocking by comparison between the conversion value V and the knock decision level V.sub.KD to control the ignition timing in accordance with the decision result.
However, as illustrated in FIG. 1, the distribution configuration of the conversion value V varies in accordance with the degree of the knocking (i.e., small knocking, middle knocking and great knocking) such that the turning point of the distribution configuration is directed to a smaller cumulative probability point as the knocking becomes greater. In addition, since the inclination of the distribution configuration is smaller in the range above the turning point, in the case that the knocking is great so that the turning point becomes below the central value V.sub.M, the value S corresponding to the standard deviation rapidly becomes large. That is, in FIG. 1, the range of the conversion value V corresponding to the cumulation 33 (%) below the central value VM in the case that the degree of the knocking is large (the range indicated by character a) considerably becomes wider as compared with the case that the degree of the knocking is middle (the range indicated by character b). This range (V.sub.M -V.sub.M /S) can be represented as V.sub.M .multidot.(1-1/S) and hence varies in accordance with the value S in the case that the central value V.sub.M does not vary greatly. Thus, it is considered as the value S is considerably great when the degree of the knocking is great so that the range becomes wide. In the case that the degree of the knocking is great to increase the value S, the knock decision level determined by S.sup.3 .times.V.sub.M also becomes high, whereby as illustrated in FIG. 2 the knock decision probability scarcely varies as compared with the case that the degree of the knocking is middle, thereby producing the result that, irrespective of the degree of the knocking being great, the retardation control is substantially effected only by the amount taken when the degree of the knocking is middle. Accordingly, the conventional knocking control system provides a problem that, in cases where the degree of the knocking is great so that the turning point in the distribution is below 50% (in the case of heavy knocking), the control to the retardation side becomes insufficient so as not to surely reduce the knocking to thereby provide the possibility that the engine is damaged.